Among petroleum products, for example, lube oils, gas oils, jet fuels, and the like are products in which fluidity at low temperatures is regarded as important. For this reason, it is desirable that base oils used for these products be such that waxy components such as normal paraffins or slightly branched isoparaffins, which are responsible for reducing the low-temperature fluidity, have been completely or partially removed, or converted to components other than waxy components. Hydrocarbons obtained by Fischer-Tropsch synthesis (hereinafter abbreviated to “FT synthesis oils”) have recently attracted attention as feedstocks for producing lube oils or fuels, because they do not contain substances of concern such as sulfur compounds; however, these hydrocarbons also contain many waxy components.
An example of a known dewaxing technique for removing waxy components from hydrocarbon oils is a method wherein waxy components are extracted using a solvent such as liquefied propane or MEK. However, this method has problems in that the operating costs are high, the types of usable feedstocks are limited, and the product yield is limited by the type of feedstock.
On the other hand, an example of a known dewaxing technique for converting waxy components in a hydrocarbon oil to non-waxy components is catalytic dewaxing in which the hydrocarbon oil is contacted, in the presence of hydrogen, with a so-called bifunctional catalyst capable of hydrogenation-dehydrogenation and isomerization, thereby isomerizing normal paraffins in the hydrocarbons to isoparaffins. Further, examples of known bifunctional catalysts used for catalytic dewaxing include catalysts containing solid acids, represented by molecular sieves made of, for example, zeolites, and metals belonging to Groups 8 to 10 or Group 6 of the periodic table; and in particular, catalysts in which these metals are supported on molecular sieves.
While catalytic dewaxing is an effective method for improving the low-temperature fluidity of hydrocarbon oils, it is necessary to sufficiently increase the normal paraffin conversion in order to obtain a fraction that is suitable as a lube base oil or fuel base oil. However, because the above-mentioned catalysts used in catalytic dewaxing are capable of both isomerization and hydrocarbon cracking, when a hydrocarbon oil is catalytic dewaxed, conversion of the hydrocarbon oil into lighter products also proceeds as the normal paraffin conversion increases, making it difficult to obtain a desired fraction in good yield. Particularly when producing a high-quality lube base oil in which a high viscosity index and low pour point are required, it is very difficult to economically obtain a desired fraction by catalytic dewaxing of a hydrocarbon oil; for this reason, synthetic base oils such as poly-alpha-olefins have been frequently used in this field.
In recent years, however, in the fields of the production of lube base oils and fuel base oils, and, in particular, in the field of the production of lube base oils, the production of Group II, Group III, and Group III+ base oils employing hydroprocessing has become increasingly popular. Under such circumstances, there is a need for a hydroisomerization catalyst having both suppressed cracking activity for hydrocarbons and high isomerization reaction activity, i.e., having excellent isomerization selectivity, for the purpose of obtaining a desired isoparaffin fraction in good yield from a hydrocarbon oil containing waxy components.
Attempts to improve the isomerization selectivity of catalysts used in catalytic dewaxing have been made in the past. For example, Patent Document 1 listed below discloses a process for producing a dewaxed lube oil, wherein a straight-chain or slightly branched hydrocarbon feedstock having 10 or more carbon atoms is contacted under isomerization conditions with a catalyst comprising a molecular sieve, such as ZSM-22, ZSM-23, or ZSM-48, having one-dimensional pores of an intermediate size and containing a metal of Group VIII or the like of the periodic table, and having a crystallite size of no more than about 0.5μ. Patent Document 2 listed below discloses a process for producing a dewaxed catalyst by modifying a zeolite such as SSZ-32 with a metal such as Ca, Cr, Mg, La, Ba, Na, Pr, Sr, K, or Nd.
It is noted that a molecular sieve that constitutes a catalyst for catalytic dewaxing is typically produced by hydrothermal synthesis in the presence of an organic template having an amino group, ammonium group, or the like, in order to construct a predetermined porous structure. The synthesized molecular sieve is then calcined in an atmosphere containing molecular oxygen at a temperature of, for example, about 550° C. or more, to thereby remove the organic template contained therein, as described in, for example, the final paragraph of the “2.1. Materials” section on page 453 of the Non-Patent Document 1 listed below. Next, the calcined molecular sieve is typically ion-exchanged into an ammonium form in an aqueous solution containing ammonium ions, as described in, for example, the “2.3. Catalytic experiments” section on page 453 of the Non-Patent Document 1. A metal components of Group 8 to 10 or the like of the periodic table is further supported on the ion-exchanged molecular sieve. The molecular sieve on which the metal component is supported is then subjected to steps such as drying, and optionally molding, and then loaded in a reactor; the molecular sieve is typically calcined in an atmosphere containing molecular oxygen at a temperature of about 400° C., and is further subjected to reduction treatment with, for example, hydrogen, at about the same temperature; consequently, the molecular sieve is provided with catalytic activity as a bifunctional catalyst.